Plasticating screw

ABSTRACT

A plasticating screw including a grating mixing section provided to enhance mixing of incompatible materials. The screw includes a mixing section including an inlet groove having a closed distal end, a discharge groove circumferentially spaced from the inlet groove, the discharge groove having a closed proximal end. Located between the inlet groove and the discharge groove is a land region is including at least one grating element that includes a plurality of grooves extending between the inlet groove and the discharge groove.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/408,980 filed Apr. 8, 2003 now U.S. Pat. No. 7,021,816 which claimsthe benefit of U.S. provisional patent application Ser. No. 60/370,781,filed on Apr. 8, 2002, the entire disclosure of which is incorporated byreference.

FIELD OF THE INVENTION

The present invention is directed as a plasticating screw, and moreparticularly at a plasticating screw configured to reduce the size ofcontaminants through melt processing.

BACKGROUND OF THE INVENTION

Thermoplastic materials are used to manufacture items that range fromfood packages to automotive body panels. In many cases, particularly incases where superior product performance or appearance is required,multi-layer or multi-material constructions are utilized. Suchapplications could include: painted plastic parts, plastic packages withbarrier coatings, clear coated plastic parts, co-extruded or co-moldeditems, multi-shot molded parts, and in-mold paint film applications.

In each of the cases listed above, the presence of the multi-materialstructure creates problems with respect to the mechanical“recyclability” the materials. The other material layers will often actas incompatible contamination, and have a generally negative impact onthe recyclability of the primary thermoplastic material. For example,most of the paints or coatings used to paint plastic parts arethermosetting in nature and act as solid particulate inclusions in therecycled matrix material. Ideally, recovered materials are fullyseparated or segregated prior to recycling. This provides the highestvalue recycle stream(s).

Methods for removing coatings from coated plastic parts have beendeveloped, but are not always cost effective. Similarly, separatingmulti-material or multi-layer structures may or may not be technicallyfeasible depending on their specific construction. However, the cost ofsegregating multi-material structures can often be more than economicsjustify. In other cases, such as in the case of a complex automotivecarpet structure, separation of the individual plastic constituents thatmake up the structure is not technically feasible. In such cases, themulti-material formulation is most easily reused as a commingled stream.In some cases, additional materials or compatibilizing agents are usedto enhance the physical properties and/or processability of thecommingled feed. As an alternative, these complex material constructionscan sometimes be effectively recycled if the contaminating layers can bereduced in particle size and effectively mixed into the continuousthermoplastic matrix material.

Studies have shown that the “degree of mixing” is an extremely importantvariable when reprocessing commingled plastic materials or acontaminated plastic formulation. The same exact material compositioncan exhibit very different physical properties if it is melt processedat different temperatures or with a different mixing history (or degreeof mixing). Mixing during compounding and/or reprocessing are bothimportant. The specific nature of the materials involved will determinewhether distributive or dispersive mixing (or both) is most important.

An example of a multi-layer plastic item is a painted thermoplasticautomotive body panel, such as a thermoplastic polyolefin bumper coatedwith a primer/paint system. In-plant (post industrial) recycling isimportant for painted parts that are rejected for quality reasons,however post consumer recycling of painted plastic automobile bodypanels becomes more important by the day. If a painted or coatedthermoplastic product is simply granulated and extruded (or injectionmolded) into a new, second generation plastic item, the end product willlikely exhibit inferior mechanical properties and surface finish. Mostpaints or coatings used in such applications are thermosetting innature, and do not re-melt when the substrate thermoplastic material isreprocessed. The un-melted paint flakes can act as contamination in therecycled plastic matrix, resulting in mechanical property-and surfacefinish quality problems.

There are a variety of ways to deal with the problems associated withreprocessing of painted or coated reground plastic. It is possible toremove the coatings from the plastic granules by methods such aschemical attack, differential thermal expansion, abrasion, etc. However,all of these techniques involve additional processes, handling,equipment and significant cost. Other methods of paint removal involvechemical degradation of the paint film and removal of the volatiledegradation products. Melt filtration has also been used to removecoatings. Many other paint removal methods have been developed usingother concepts such as chemical stripping, autoclave treatment,differential thermal expansion and pulverization.

One possible alternative to the decontamination techniques describedabove is to reuse the contaminated plastic without removing the paintcoating or other non-melting contaminate. Eliminating the pre-processingpaint removal steps would reduce the overall reprocessing costs duringrecycling, but unfortunately these cost reductions are at the expense ofmaterial performance or quality. However, there are a number ofsecondary recycling applications where the physical properties of thecontaminated material may still be adequate for the application. Onefactor that is known to be important for contaminated plastics is thephysical “size” and “shape” of the contaminating particles. Generally, asmaller particle, with more uniform particle size distribution, resultsin a more homogeneous material. Particle size reduction for solidcontaminates can be achieved through granulation, or through some typeof melt mixing action. Molded plastic parts normally have a wallthickness in the 1.0 mm to 3.0 mm range. A typical granulator used forgranulating painted plastic parts uses a 10 mm diameter dischargescreen. The granules (and paint coating) discharged from the granulatorcan have length and width dimensions as large as this screen holediameter. Therefore, size of the paint flakes (i.e. their width andlength) associated with granulated plastics are typically greater thanthe thickness of a typical molded plastic part. It is expected that thesurface appearance of molded parts produced from reground paintedplastics would be improved if the contaminating flakes have a smallerphysical size. Smaller size contamination will also cause fewer problemsif the contaminated melt flows through thin wall sections or smallorifices (such as a hot runner gate). The physical size of the paintflake contamination can be reduced by regrinding the painted parts witha granulator having a smaller discharge screen diameter. However, thisapproach presents problems with both granulator throughput rates, andpossible material handling difficulties (fines and powder).

SUMMARY OF THE INVENTION

A plasticating screw consistent with the present invention includes aflighted region at a proximal of the screw and a mixing section adjacenta distal end of the screw. The mixing section includes an inlet groovehaving a closed distal end and a discharge groove having a closedproximal end, with the discharge groove being circumferentially spacedfrom the inlet groove. The region between the inlet groove and thedischarge groove is a land region having at least one grating elementincluding a plurality of grooves extending between the inlet groove andthe discharge groove.

A mixing section for a plasticating screw is provided including an inletgroove having a closed distal end and a discharge groovecircumferentially spaced from the inlet groove, the discharge groovehaving a closed proximal end. Located between the inlet groove and thedischarge groove is a land region including at least one grating elementcomprising a plurality of grooves extending between said inlet grooveand said discharge groove.

The present invention also provides a method for mixing a primarythermoplastic material with a second material including providing anextruder having a screw comprising a mixing section including an inletgroove and a discharge groove having disposed therebetween at least onegrating element including a plurality of grooves extending between saidinlet groove and said discharge groove, providing a combination of saidprimary material and said second material to said extruding, and meltextruding the combination of said primary material and said secondmaterial using said extruder, whereby a domain size of said secondmaterial is reduced after melt extruding.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of the present invention are described in thefollowing detailed description of exemplary embodiments, whichdescription should be understood in conjunction with the accompanyingdrawings wherein:

FIG. 1 illustrates a portion of an exemplary plasticating screw having amixing section consistent with a first embodiment of the presentinvention;

FIG. 2 is a cross-sectional view of the grating or land portion of theexemplary mixing section illustrated in FIG. 1;

FIG. 3 illustrates a portion of an exemplary plasticating screw having amixing section consistent with a second embodiment of the presentinvention;

FIGS. 4 a and 4 b, respectively, are cross-sectional views of the firstand second grating land elements of the mixing section illustrated inFIG. 3;

FIG. 5 illustrates a portion of an exemplary plasticating screwincluding a mixing section consistent with a third embodiment of thepresent invention;

FIGS. 6 a and 6 b, respectively, are cross-sectional views of the firstand second grating land elements of the third exemplary mixing sectionillustrated in FIG. 5; and

FIGS. 7 a through 7 c show scanned images extruded thermoplastic olefinresin including thermoset paint particles produced using (a) generalpurpose screw, (b) UC mixer, and (c) two row grating mixer consistentwith the present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENT

The present invention relates to plasticating screw and a mixing sectionfor a plasticating screw that may advantageously reduce the domain sizeof secondary material in a primary material matrix. Consistent with thepresent invention, the secondary material is a material that may formdiscrete domains within the primary material matrix. Accordingly, thesecondary material may include thermoplastic materials that are notcompatible with the primary material, materials solid or gel material,for example thermoset materials, or thermoplastic materials having ahigher melting point than the primary material. The secondary materialmay also include various other polymeric and non-polymeric materialsthat do not readily mix with the primary material.

Referring to FIG. 1, a portion of a plasticating screw 100 consistentwith a first exemplary embodiment is shown. The plasticating screw 100may generally be of the variety used in single screw extrusion ofthermoplastic materials and the like. The screw 100 may include a root102 having at least one flight 104 helically wound there-around forconveying and shearing the thermoplastic material. Additionally, thescrew 100 includes a mixing section 106.

The mixing section 106 generally includes a region having an enlargeddiameter D that may generally correspond to the diameter of the flight104. The mixing section 106 further includes a plurality ofcorresponding, adjacent inlet grooves 108 and discharge grooves 110. Theinlet grooves 108 are configured accept a flow of material from the feedzone of the extruder. However, the inlet grooves 108 are closed at thedistal end, preventing axial transfer of material therefrom. In acorresponding manner, the discharge grooves 110 have a closed proximalend and an open distal end thereby permitting axial transfer of materialfrom the discharge groove 110 toward the nozzle of the extruder. Theinlet groove 108 and discharge groove 110 may each be generally axiallyaligned with the screw 100. Alternatively, the inlet groove 108 anddischarge groove 110 may be angled relative to the axis of the screw100.

The inlet grooves 108 and discharge grooves 110 are separated by agrating land element 112. Referring to FIG. 2, a cross-sectional view ofthe land 112 is shown including a plurality of openings 114 and teeth116 oriented generally perpendicular to the axis if the inlet grooves108 and discharge grooves 110. According to a first exemplaryembodiment, the openings may have a width in the range of about 0.002 to0.200 inches and a depth in the range of about 0.002 to 0.060 inches.

In FIG. 1 the anticipated movement of material through the mixingsection is indicated by bold arrows. As shown, when the screw isoperated rotating in a clockwise direction as viewed from the nozzle ofthe extruder material will be conveyed along the screw 100 to the mixingsection 106. The material will enter the inlet grooves 108 and besmeared across the land 112 into the discharge groove 110. From thedischarge groove 110 the material may be further transferred to thenozzle of the extruder. In addition to “smearing” material across theland 112, the teeth 114 inflict a grating mechanism on the material.

In the presence of a second material making up discrete domains, thegrating mechanism of the land 112 may reduce the domain size of thesecond material as well as improve mixing of the materials beingextruded. In the exemplary embodiment depicted in FIG. 1, the secondmaterial 120 generally travels through the extruder carried by theprimary material. As illustrated, the second material 120 passes intothe inlet grooves 108 and then is transferred across the grating landelements 112 to the discharge grooves 110. As shown, transferring thesecond material 120 across the land elements 112 acts to reduce thedomain sized of the second material 120 b. The reduced size secondmaterial 120 b is discharged from the discharge groove 110 towards thenozzle of the extruder.

It should be appreciated that the effects of the grating action of theland 112 are applicable to particulate, gel, molten, and semi-moltenmaterials, as well as combinations thereof. Additionally, theplasticating screw herein is suitable for use with mixtures of materialsincluding more than two components.

Turning to FIG. 3 a portion of a second exemplary plasticating screw 300is shown having the same general configuration as the first exemplaryscrew. Similar to the first embodiment the screw 300 includes a mixingsection 306 disposed between two flighted regions 302 and 304 of thescrew. The mixing section 306 includes adjacent pairs of inlet grooves308 and discharge grooves 310. Each associated inlet groove 308 anddischarge groove 310 is separated by a land element 312, generally.

The land element 312 of the second embodiment include three elements: afirst grating element 314 adjacent to the inlet groove 308, a secondgrating element 318 adjacent the discharge groove 310 and a closed-endedseparating channel or groove 316 disposed between the first and secondgrating elements 314, 318.

Referring to FIGS. 4 a and 4 b, the first and second grating elements314, 318 may include similarly spaced groove/teeth arrangements 330/332and 340/342 respectively. In this manner, the second mixing section 306is generally the same as the first mixing section 106, only having agroove or channel in the approximate middle of the land. Alternatively,the groove/teeth of the respective first and second grating elements314, 318 may be offset such that any groove 330 in the first gratingelement 314 is aligned with a tooth 342 of the second grating element318.

In operation, a second material 320 carried by a primary materialtraveling through the mixing section 306 enters the inlet groove 308 andis transferred across the first grating element 314 and into theseparating channel 316. The second material 320 is then transferred fromthe separating channel 316 to across the second grating element 318 tothe discharge groove 310. Being passed across the grating elementsreduces the domain sizes of the second material and improvesdispersion/distribution of the second material in the primary material.The two grating elements 314, 318 provide two grating cycles to everydomain of the second material 320 conveyed by the screw 300, whichprovides a narrower domain size distribution of the second material.

FIG. 5 illustrates a third exemplary plasticating screw 500 consistentwith the present invention including a mixing section 506. The mixingsection 506 includes pairs of adjacent inlet grooves 508 and 510separated by a land region 512, generally. As with the previousembodiment, the land 515 includes two grating elements 514, 518separated by a channel or groove 516. Differing from the previousembodiment, however, the second grating element 518 of this embodimentis finer than the first grating element 514. This aspect is illustratedin FIGS. 6 a and 6 b which show cross-sections of the first and thesecond grating elements 514, 518 respectively. As shown in FIGS. 6 a and6 b, the grating of the second element 518 may be finer than the firstelement 514 both in the width and depth of the grating. Alternatively,it may be desirable to reduce only one aspect of the grating, i.e. onlyeither the width of the depth.

It should be appreciated that the land region between each associatedinlet groove and discharge groove may be divided into any number ofgrating elements, which grating elements may be programmed with anyvariety of profiles. That is, the several grating elements may be of thesame dimensions, or may be of decreasing or increasing coarseness orcombinations thereof.

Additionally, it should be understood that the design and configurationof the flighted sections of the plasticating screw are largelyirrelevant to the present invention. The mixing section may be equallysuitable for use with single flighted and multi-flighted screws havingconstant or variable pitch, etc. Additionally, while it may beadvantageous to locate the mixing section in the transition or meteringsections of the screw, the exact location of the mixing section may beadjusted to suit individual applications without departing from theinvention herein.

At the time of filing, it is believed that the effectiveness of thegrating mixer according to the present invention is most likelyassociated with the entrance region(s) of the grating rows. Theexperimental studies with this grating concept have shown that two shortland grating rows are significantly more effective than one longer landgrating row of the same cross section. Additional benefits would beexpected if additional rows, especially offset rows, were added.However, appreciable advantages in contaminant size reduction may berealized from a mixing section including only one grating element perinlet/discharge pair.

EXPERIMENTAL EXAMPLES

The degree of paint flake size reduction during extrusion provided by aplasticating screw having a mixing section consistent with the presentinvention was comparatively evaluated against a conventional squarepitch screw without a mixing section and a plasticating screw having aconventional UC mixing section, in each case using a 38 mm diameter 24L/D Welex single screw, having an 8.0 mm deep feed section andinterchangeable 7 L/D removable discharge end.

For the experimental evaluation, the interchangeable discharge end ofthe screw was alternatively configured to have a 3.2 mm deep meteringsection (the conventional or general purpose screw), a 2 L/D UC (LeRoymixing section screw) mixing section followed by a 3 turn meteringsection, and a 2 L/D double grating element mixing section consistentwith the present invention. The UC mixing section was provided havingthree 0.50 mm deep×9.6 mm long×2 L/D wide shear lands. The grated mixingelement according to the present invention was provided having three,double grating element land regions located at the same axial positionalong the screw, but 120° apart from one another. Each land region hadtwo rows of grating elements, one at the inlet, and one at the dischargeof the shear zone. Each grating element had 36 narrow, shallow openings,with the width and depth of each grating slot (for the screw sectionevaluated in this study) are 0.81 mm and 1.5 mm respectively.

The material utilized in comparative evaluation was reground, paintedthermoplastic olefin (TPO) automobile bumpers. The TPO itself had ablack color, and was coated with a thermoset paint (of various colors,on one side of the granules). The base TPO resin used for these autobumpers was a Solvay Sequel® 1440. Virgin (unpainted) TPO of the samegrade was also utilized in this study as a control. The virgin TPO had amelt flow rate of 8.3 g/10 min @ 190° C., 2.16 Kg (as measuredexperimentally). The granulated material was produced from parts havingan average wall thickness in the range of 2.5 mm. A sieve analysis wasconducted on the granulated material after removing a small percentageof very large shreds. It should be appreciated that such a sieveanalysis on granulated parts will generally indicate the narrower of thex-y dimensions; found to be 4.6 mm as received.

Experiment 1

With each of the three screw configurations, the screws were used toplasticate the melt at a screw speed of 80 RPM, and barrel temperaturesset from 210° C. (feed) to 224° C. (discharge). In the firstexperimental evaluation, the extruder was operated under “open dischargeconditions”, i.e., without a screen pack breaker plate, or die. The sameprocess conditions were used for all trials. Output, melt temperature(via hand held TC) and drive variables were recorded for each trial. Inaddition, golf ball size samples of the molten extrudate obtained witheach screw configuration were placed in a heated compression presslocated next to the extruder. The melt was compressed into very thinfilms having a thickness of 0.10 mm and cooled. The size and sizedistribution of the paint flakes where evaluated by making atransparency from scanned images of the films, and projecting an imageof the films onto large paper screens using a high magnificationprojector. The longest dimension of each paint particle (projected)image was measured and marked with a pen in order to avoid doublecounting. These measurements were related to the actual dimension of thepaint flake using a calibration based on the magnification of theprojector. The average size and size range of the measured paint flakesbased on the flake's long dimension measurements is reported in Table 1below.

Experiment 2

In a second comparative evaluation, the same three screw configurationsfrom Experiment 1 were used to extrude the painted TPO regrind at thesame conditions as in Experiment 1. In Experiment 2 a pelletizing(strand) die was added to the extruder, however, again no breaker plateor screens were used (only an open seal ring was used). Pressed filmshaving a thickness of 0.10 mm were produced for paint flake sizeanalysis, as in Experiment 1. Additionally, quantities of pelletizedmaterial were produced for subsequent injection molding. The pelletizedmaterials were injection molded using a 110 Ton Milacron Electra (allelectric) injection molding machine into standard ASTM test specimenshaving a thickness of 3.2 mm. The injection molded test specimens testedto determine if the paint flake size distribution had any affect on thecontaminated material's (or molded part) physical properties andappearance. While some additional paint flake size reduction wouldlikely occur during pelletization and molding, the molding conditionswere manipulated to minimize any additional process related damage.Specifically, a general purpose injection molding screw with a largediameter nozzle tip, and wide, full thickness gates were used. Theinjection molding conditions were also kept as constant as possible forall trials in an effort to “zero out” the additional paint flake damage.The virgin TPO pellets were also extruded, re-pelletized, and molded asa control for this study.

Experiment 3

Experiment 3 also used the same three screw configurations and extruderand conditions as the previous two experiments. In Experiment 3 both abreaker plate/screen pack (20-40-60-100-20 screens) and a pelletizing(strand) die were used. This experiment was performed to evaluate theinfluence of screens on paint flake size distribution and concentrationin the melt, as well as molded part quality. 0.10 mm thick films wereprepared for paint flake size analysis, as in the preceding experiments,and quantities of pelletized material were produced for subsequentinjection molding. These “melt filtered” pelletized materials wereinjection molded using the same equipment and procedure described inExperiment 2 above. Once again, the injection molding conditions werekept constant for all trials in an effort to zero out any additionalpaint flake damage.

Experimental Results

Referring to FIGS. 7 a through 7 c, scanned images of representativefilms produced using the conventional metering screw, UC mixing section,and grated mixing section according to the experimental screwconfigurations are respectively shown. All images are at the same scale.The results are of paint flake size and distributions are also presentedin Table 1, below. As can be seen from both the scanned images and thetabulated data, the average paint flake size obtained using the doublerow grating screw of the present invention is significantly smaller thanthose obtained using either the general purpose or LeRoy screws. Thevery largest paint flake length observed with the double row gratingmixer was 47% smaller than that observed using the general purposescrew. The lower range value was also about 50% smaller. The physicalsize of the paint flakes is smaller, and as a result, there are more offlakes. Table 1 indicates that the smallest average particle length wasgenerated with the double row grating screw.

Furthermore, it can be seen that there was little difference in paintflake size or size distribution for the films produced using the generalpurpose screw and LeRoy mixing section screw. The films produced usingthe general purpose screw and the LeRoy mixing section had wide paintflake length distributions, as indicated visually from the films, by thelarge standard deviation values, and by the very large range values.Some relatively large paint flake sizes were clearly visible for all ofthe films produced from melt that was plasticated with these screws.

Observed output, average melt temperature (using a preheatedthermocouple melt probe), and motor current were recorded for each screwat 80 RPM from the above experimental examples is presented in Table 2.The outputs observed for each screw were essentially the same in eachcase. The addition of LeRoy mixer or double row grating mixer did nothave a significant effect on output. However, they did have some effecton melt temperature and drive motor current. The bulk melt temperaturefor the LeRoy screw was 8° C. higher than that of the GP screw, a 3.5%increase. The bulk melt temperature for grating screw was 14° C. higherthan that of the GP screw, a 6% increase. The increase in drive currentreflects these melt temperature increases.

While, based on the increased melt temperature and shear history for themixing screws, there was some concern that thermal or shear degradationcould have taken place. However, an examination of melt index ofre-pelletized material produced using each respective screw indicated nosignificant thermal or shear induced degradation associated with eitherof the two mixing sections as compared to the general purpose screw. Themelt index testing was conducted on melt filtered samples to minimizepaint flake interference. The average results (3 replicates) of the meltindex test conducted at 190° C., 2.16 kg for the general purpose screw,LeRoy mixing section, and grated mixing section were 7.63 g/10 min.,7.57 g/10 min., and 7.63 g/10 min., respectively.

The physical property test results for injection molded samples producedusing palletized extrudate from each of the three screw configurations,as well as virgin thermoplastic olefin control material are shown inTable 3. The test results indicate that ultimate elongation is thephysical property most sensitive to particulate contamination. Theextruded and molded virgin pellets had an ultimate elongation of 34% ata test rate of 50 mm/min. The (directly) molded TPO painted regrind hadthe lowest ultimate elongation value of 12%. When the painted regrindthat was re-pelletized without the use of screens, the ultimateelongation values obtained were in the 20% range. The type of screw usedfor extrusion/pelletizing did not appear to be a significant factor.However, qualitatively, the appearance of the molded parts was visiblyimproved for the re-pelletized material produced with the grating screwbased only on visual observation.

1. A plasticating screw including a root surface comprising: a flightedregion at a proximal end of the screw; a mixing section adjacent adistal end of the screw, said mixing section comprising an inlet groovehaving a closed distal end, a discharge groove circumferentially spacedfrom said inlet groove, said discharge groove having a closed proximalend, and a grooved land region comprising at least one grating landelement extending between said inlet groove and said discharge groove;wherein said inlet groove and said outlet groove include an axis;wherein said grating land element includes a plurality of openings andteeth oriented generally perpendicular to said axis of said inlet andsaid discharge grooves, wherein said openings do not extend to said rootsurface; and wherein said grooved land region has a diameter thatgenerally corresponds to that of said flighted region.
 2. A plasticatingscrew according to claim 1, wherein said mixing section comprises threesets of associated inlet grooves and discharge grooves separated by atleast one grating element.
 3. A plasticating groove according to claim 1wherein each of the plurality of openings of the at least one gratingland element have a width that is in the range of between about0.002-0.200 inches and a depth that is in the range of between about0.002-0.060 inches.
 4. A plasticating screw according to claim 1 whereineach of the plurality of openings of the at least one grating landelement have a width of about 0.032 inches and a depth of about 0.059inches.
 5. A mixing section for a flighted plasticating screw whichincludes a root surface, comprising an inlet groove having a closeddistal end, a discharge groove circumferentially spaced from said inletgroove, said discharge groove having a closed proximal end, and agrooved land region comprising at least one grating land elementextending between said inlet groove and said discharge groove; whereinsaid inlet groove and said outlet groove include an axis; wherein saidgrating land element includes a plurality of openings and teeth orientedgenerally perpendicular to said axis of said inlet and said dischargegrooves, wherein said openings do not extend to said root surface; andwherein said grooved land region has a diameter that generallycorresponds to that of the flighted region.
 6. A mixing section for aplasticating screw according to claim 5, wherein said mixing sectioncomprises three sets of associated inlet grooves and discharge groovesseparated by at least one grating element.
 7. A mixing section for aplasticating screw according to claim 5 wherein each of the plurality ofopenings of the at least one grating land element have a width that isin the range of between about 0.002-0.200inches and a depth that is inthe range of between about 0.002-0.060 inches.
 8. A mixing section for aplasticating screw according to claim 5 wherein each of the plurality ofopenings of the at least one grating land element have a width about0.032 inches and a depth of about 0.059 inches.
 9. A method for mixing aprimary thermoplastic material with a second material comprising:providing an extruder having a flighted screw which includes a rootsurface, comprising a mixing section including an inlet groove and adischarge groove having disposed therebetween a grooved land regioncomprising at least one grating land element extending between saidinlet groove and said discharge groove wherein said grooved land regionhas a diameter that generally corresponds to that of the flightedregion; wherein said inlet groove and said outlet groove include anaxis; wherein said grating land element includes a plurality of openingsand teeth oriented generally perpendicular to said axis of said inletand said discharge grooves, and wherein said openings do not extend tosaid root surface; providing a combination of said primary material andsaid second material to said extruder and melt extruding the combinationof said primary material and said second material using said extruder,whereby a domain size of said second material is reduced after meltextruding.